1. Field of the Invention
This invention relates to frequency-responsive circuitry. More particularly, this invention relates to such circuitry for determining when the frequency of an alternating signal, such as a clock signal, is below a predetermined level.
2. Description of the Prior Art
Lap top computers can be powered either with internal battery, i.e., when operating in stand-alone mode with the on-board black-and-white, low-power display device they carry, such as a liquid crystal display (LCD), or can be plugged into line power for operation with an auxiliary higher-power-consumption three-color display monitor providing a superior presentation of data but requiring control circuitry which consumes power at a rate which would quickly exhaust an on-board battery.
When in battery-powered mode, much of the display control circuitry of the computer is not needed for the relatively simpler black-and-white display, and those portions of the computer circuitry can be powered-down in that mode in order to conserve battery power. The computer advantageously is provided with automatic means for turning off such power-consuming circuitry as is not needed when operating with the battery-powered black-and-white display.
Some three-color monitor displays may require a clock frequency of from about 25 to 40 MHz. System considerations, such as testing, may dictate the use of much lower clock frequencies when operating on a-c line power, and a limit of 200 kHz might for example be selected as the lower threshold for line-power operation. When operating with battery power for the on-board black-and-white display, the clock frequency of the color display circuitry is set to be much lower, or even at zero, because of the relative simplicity of the on-board display. Control means responsive to clock frequency thus can be used to determine when the computer is being operated on battery power. Following such determination, the control means will automatically turn off the unneeded circuits. To be commercially practical, such control means must be highly reliable in operation, yet relatively inexpensive to make.